1. Field of the Invention
The present invention relates generally to step-down switching regulators of a synchronous rectification type capable of changing output voltage, and more particularly to a step-down switching regulator having a circuit to interrupt a backflow of current from an inductor for smoothing to a switching device for synchronous rectification by turning off the switching device upon detection of the backflow of current.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing a conventional step-down switching regulator 100.
According to the switching regulator 100, a supply voltage supplied from a DC power supply such as a battery is input to an input terminal IN as an input voltage Vin, and a predetermined constant voltage is generated from the input voltage Vin and output from an output terminal OUT to a load 111 as an output voltage Vout.
The switching regulator 100 includes a switching device SWa to perform output control of the input voltage Vin, a diode Da forming a flywheel diode, an inductor La and a capacitor Ca for smoothing, and resistors Ra and Rb for output voltage detection. The resistors Ra and Rb generate a divided voltage Vfb by dividing the voltage Vout output to the load 111, and output the divided voltage Vfb.
The switching regulator 100 further includes a reference voltage generator circuit 102, an error amplifier circuit 103, and a triangle wave generator circuit 104. The reference voltage generator circuit 102 generates and outputs a predetermined reference voltage Vref. The error amplifier circuit 103 compares the divided voltage Vfb and the reference voltage Vref, and amplifies and outputs the voltage difference therebetween. The triangle wave generator circuit 104 generates and outputs a predetermined triangle wave signal TW. The switching regulator 100 further includes a PWM (Pulse Width Modulation) comparator circuit 105 and a drive circuit 106. The PWM comparator circuit 105 generates and outputs a pulse signal for PWM control corresponding to the voltage difference between the output voltage of the error amplifier circuit 103 and the triangle wave signal TW from the triangle wave generator circuit 104. The drive circuit 106 controls switching of the switching device SWa in accordance with the pulse signal from the PWM comparator circuit 105.
When the switching device SWa performs switching to turn ON, a current is supplied to the inductor La. When the switching device SWa turns OFF, energy stored in the inductor La is released through the diode Da. A current generated at this point is smoothed by the capacitor Ca and output from the output terminal OUT to the load 111. Further, the output voltage Vout output from the output terminal OUT is divided between the resistors Ra and Rb, and the divided voltage Vfb is input to the inverting input of the error amplifier circuit 103.
When the output voltage Vout of the switching regulator 100 increases, the output voltage of the error amplifier circuit 103 decreases, so that the duty cycle of the pulse signal from the PWM comparator circuit 105 is reduced. As a result, the ON-time of the switching device SWa is reduced, so that the output voltage Vout of the switching regulator 100 is controlled so as to decrease. On the other hand, when the output voltage Vout of the switching regulator 100 decreases, an operation opposite to the operation described above is performed. As a result, the output voltage Vout of the switching regulator 100 is controlled so as to be constant.
Thus, it is possible to change the output voltage of a switching regulator by changing a reference voltage (for example, see Japanese Laid-Open Patent Application No. 2001-161063).
On the other hand, according to a step-down switching regulator of a synchronous rectification type in which a switching device SWb for synchronous rectification is provided in place of the diode Da shown in FIG. 1 so that the switching devices SWa and SWb are controlled to switch ON and OFF complementarily, at a light load time when the resistance of the load 111 increases, the direction of a current flowing through the inductor La is reversed so that a current flowing from the inductor La to ground through the switching device SWb is generated, thereby causing a decrease in efficiency. In order to avoid this decrease in efficiency at a light load time, it is effective to include a control circuit that detects such a backflow of current and turns OFF the switching device SWb upon detection of the backflow of current.
However, in the synchronous rectification step-down switching regulator capable of changing output voltage, it is necessary to change the output voltage swiftly even at a light load time, so that it is difficult to interrupt a backflow of current by turning OFF the switching device SWb upon detection of the backflow of current. This causes the problem of reduced efficiency at a light load time.